If you haven't already bought a TiBook, don't. They're very pretty, but they're a pain to take apart and maintain and they're quite physically unreliable. I also never found them to be faster at Mac OS 9 (or really, even at OS X) than the PowerBook G3. The one exception is if I created a really huge scene in a 3d rendering app, but even then, that's the kind of thing that if you can swing it, you should do on some kind of desktop anyway.
Also, if you need an optical drive for a PPC Mac, spend the money on a firewire one. It's not easy or convenient to boot from a USB optical drive on a PPC Mac, even if they work for other tasks.
In terms of OS X as "retrochallenge:" *yawn*
One of the things that has bothered me about Mac OS X on late PowerPC hardware for a really long time (before I became aware of the
security concerns (please dont' reply to those insanely old threads)) is that it's not really that different from Mac OS X on Intel-based hardware, and to be honest, I
really don't think it's a very big challenge.
Yeah, you're dealing with the fact that PPC hardware is slow and insufficient for the absolute dog that is Mac OS X, and that it also happens to be poorly supported by slimmer, more efficient, and better operating systems, such as Linux and BSD.
To me, a real "retrochallenge" (and even this isn't very exciting) would be to go all the way back to Mac OS 9 (preferably on a G3 of some sort, but that's a personal preference) or to a really early PowerPC computer, something that won't even run Mac OS 9 very well.
The first retrochallenge had a single simple rule: ten years or older. Unfortunately today that means I could put Windows 8.1U1 on a Pentium 4 and just do all the things I do now, on a bigger and louder computer, slightly slower computer.
The current retrochallenge stuff (which seems to focus less on Macs, in part because fewer Mac users are interested in it) is focused a lot more on doing creative things with old computers, and it appears that most people believe the "endurance" challenge to be pretty boring.